Method of cross color intra prediction

ABSTRACT

In one implementation, a method operates by receiving neighboring reconstructed first-color pixels and current reconstructed first-color pixels of a current first-color block and receiving neighboring reconstructed second-color pixels of a current second-color block collocated with the current first-color block. The method then determines linear model (LM) parameters according to a linear model for one or more LM Intra modes. The method then receives input data associated with current second-color pixels of the current second-color block and generates a cross-color Intra predictor from the current reconstructed first-color pixels of the current first-color block using the LM parameters associated with a LM Intra mode selected from said one or more LM Intra modes. Finally, the method applies cross-color Intra prediction encoding or decoding to the current second-color pixels of the current second-color block using the cross-color Intra predictor for the selected LM Intra mode.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 14/655,447, filed Jun. 25, 2015, which was a National StageApplication of PCT Patent Application No. PCT/CN2014/073395, filed onMar. 13, 2014, which claimed priority to U.S. Provisional PatentApplication Ser. No. 61/805,310, filed on Mar. 26, 2013, entitled“Improved Chroma LM Mode”. The U.S. Provisional Patent Application ishereby incorporated by reference in its entirety.

TECHNICAL FIELD

The present invention relates to video coding. In particular, thepresent invention relates to coding techniques associated with Intraprediction using inter-color linear mode based on reconstructed pixelsof another color.

BACKGROUND

Motion compensated inter-frame coding has been widely adopted in variouscoding standards, such as MPEG-1/2/4 and H.261/H.263/H.264/AVC. Whilemotion-compensated inter-frame coding can effectively reduce bitrate forcompressed video, Intra coding is required to compress the regions withhigh motion or scene changes. Besides, Intra coding is also used toprocess an initial picture or to periodically insert I-pictures orI-blocks for random access or for alleviation of error propagation.Intra prediction exploits the spatial correlation within a picture orwithin a picture region. In practice, a picture or a picture region isdivided into blocks and the Intra prediction is performed on a blockbasis. Intra prediction for a current block can rely on pixels inneighboring blocks that have been processed. For example, if blocks in apicture or picture region are processed row by row first from left toright and then from top to bottom, neighboring blocks on the top andneighboring blocks on the left of the current block can be used to formIntra prediction for pixels in the current block. While any pixels inthe processed neighboring blocks can be used for Intra predictor ofpixels in the current block, very often only pixels of the neighboringblocks that are adjacent to the current block boundaries on the top andon the left are used.

The Intra predictor is usually designed to exploit spatial features inthe picture such as smooth area (DC mode), vertical line or edge,horizontal line or edge and diagonal line or edge. Furthermore, spatialcorrelation often exists between the luminance (luma) and chrominance(chroma) components. Therefore, reconstructed luma pixels can be used toderive the Intra chroma prediction. In the emerging High EfficiencyVideo Coding (HEVC), a chroma Intra prediction mode based on thereconstructed luminance signal has been considered. This type of chromaIntra prediction is termed as Linear Model (LM) prediction. FIG. 1illustrates the Intra prediction derivation for LM mode. First, theneighboring reconstructed pixels (indicated by circles) of a collocatedluma block (i.e., Y block) and the neighboring reconstructed pixels(indicated by circles) of a chroma block (i.e., U or V block) in FIG. 1are used to derive the linear model parameters between the blocks. Thepredicted pixels of the chroma block are generated using the parametersand the reconstructed pixels of the luma block. In the parametersderivation, the top reconstructed pixel row adjacent to the top blockboundary of the current luma block and the left reconstructed pixelcolumn adjacent to the left block boundary of the current luma block areused. It is noted that the second left reconstructed pixel column fromthe left boundary is used instead of the left column immediatelyadjacent to the left boundary in order to match the sampling locationsof the chroma pixels. The specific row and column of the luma block areused in order to match the 4:2:0 sampling format of the chromacomponents. While FIG. 1 illustrates the example of LM chroma mode forthe 4:2:0 sampling format, the LM chroma mode for other chroma samplingformat may also derived similarly.

According to the LM prediction mode, the chroma values are predictedfrom reconstructed luma values of a collocated block. The chromacomponents may have lower spatial resolution than the luma component. Inorder to use the luma signal for chroma Intra prediction, the resolutionof the luma signal may have to be reduced to match with that of thechroma components. For example, for the 4:2:0 sampling format, the U andV components only have half of the number of samples in vertical andhorizontal directions as the luma component. Therefore, 2:1 resolutionreduction in vertical and horizontal directions has to be applied to thereconstructed luma samples. The resolution reduction can be achieved bydown-sampling process or sub-sampling process.

In LM chroma mode, for a to-be-predicted chroma sample V with itscollocated reconstructed luma sample V_(col), the linear model togenerate LM predictor P is formulated as follows:P=a·V _(col) +b

In the above equation, a and b are referred as LM parameters. The LMparameters can be derived from the neighboring reconstructed luma andchroma samples around the current block so that the parameters do notneed to be coded in the bitstream. After deriving the LM parameters,chroma predictors can be generated from the collocated reconstructedluma samples in the current block according to the linear model. Forexample, if the video format is YUV420, then there are one 8×8 lumablock and two 4×4 chroma blocks for each 8×8 coding unit, as shown inFIG. 1., In FIG. 1, each small square corresponds to one pixel in thecurrent coding unit (2N×2N for luma and N×N for chroma) to be coded. TheLM parameters are derived first based on neighboring reconstructedsamples of the current coding unit, which are represented as circles inFIG. 1. Due to the YUV420 sampling format, the collocated chromaposition is located between two corresponding vertical luma samples. Anaverage value between two corresponding vertical luma samples is used toderive the LM parameters. For neighboring pixels above the top blockboundary, the average value is replaced by the closest sample in thevertical direction in order to reduce the line buffer requirement. Theneighboring pixels (as shown in circles) of the currently luma (Y) andchroma (U or V) coding units are used to derive the LM parameters forthe respective chroma component as shown in FIG. 1. After the LMparameters are derived, the chroma predictors are generated based on thelinear model and the collocated luma reconstructed samples. According tothe video format, an average luma value may be used instead of thecorresponding luma sample.

A method of chroma Intra prediction using extended neighboring pixelsfor LM parameter derivation has been disclosed by Zhang et al., (“NewModes for Chroma Intra Prediction”, in Joint Collaborative Team on VideoCoding (JCT-VC) of ITU-T SG16 WP3 and ISO/IEC JTC1/SC29/WG11, 7thMeeting: Geneva, CH, 21-30 Nov., 2011, document: JCTVC-G358). FIG.2A-FIG. 2C illustrate an example of chroma Intra prediction for 8×8chroma block using extended neighboring pixels according to Zhang. FIG.2A corresponds to regular chroma Intra prediction being considered byHEVC. FIG. 2B illustrates the example of LM parameter derivation basedfor an additional chroma Intra mode using extended horizontalneighboring pixels, where additional N pixels from the upper-rightneighbor are used. FIG. 2C illustrates the example of LM parameterderivation based for another additional chroma Intra mode using extendedvertical neighboring pixels, where additional N pixels from thelower-left neighbor are used. While the method of Zhang demonstratesnoticeable improvement in performance, the method also causes increasesin computational complexity and buffer requirement.

It is desirable to develop improved method that may further improve theperformance and/or reduce the buffer requirement of chroma Intraprediction.

SUMMARY

A method for cross-color Intra prediction based on reconstructed pixelsof another color using a linear model (referred as LM mode or LM Intramode) is disclosed. The method receives neighboring reconstructedfirst-color pixels and current reconstructed first-color pixels of acurrent first-color block and receives neighboring reconstructedsecond-color pixels of a current second-color block collocated with thecurrent first-color block. The method then determines linear model (LM)parameters according to a linear model for one or more LM Intra modes,wherein the LM parameters for at least one of said one or more LM Intramodes is determined based on multiple rows of the neighboringreconstructed second-color pixels adjacent to a top boundary of thecurrent second-color block and a first part of the neighboringreconstructed first-color pixels, or multiple columns of the neighboringreconstructed second-color pixels adjacent to a left boundary of thecurrent second-color block and a second part of the neighboringreconstructed first-color pixels. The method then receives input dataassociated with current second-color pixels of the current second-colorblock and generates a cross-color Intra predictor from the currentreconstructed first-color pixels of the current first-color block usingthe LM parameters associated with a LM Intra mode selected from said oneor more LM Intra modes. Finally, the method applies cross-color Intraprediction encoding or decoding to the current second-color pixels ofthe current second-color block using the cross-color Intra predictor forthe selected LM Intra mode.

In another embodiment, an apparatus of cross-color Intra predictionbased on reconstructed pixels of another color component, the apparatuscomprising one or more circuits configured to: receive neighboringreconstructed first-color pixels and current reconstructed first-colorpixels of a current first-color block; receive neighboring reconstructedsecond-color pixels of a current second-color block collocated with thecurrent first-color block; determine linear model (LM) parametersaccording to a linear model for one or more LM Intra modes, wherein theLM parameters for at least one of said one or more LM Intra modes isdetermined based on multiple rows of the neighboring reconstructedsecond-color pixels adjacent to a top boundary of the currentsecond-color block and a first part of the neighboring reconstructedfirst-color pixels, or multiple columns of the neighboring reconstructedsecond-color pixels adjacent to a left boundary of the currentsecond-color block and a second part of the neighboring reconstructedfirst-color pixels; receive input data associated with currentsecond-color pixels of the current second-color block; generate across-color Intra predictor from the current reconstructed first-colorpixels of the current first-color block using the LM parametersassociated with a LM Intra mode selected from said one or more LM Intramodes; and apply cross-color Intra prediction encoding or decoding tothe current second-color pixels of the current second-color block usingthe cross-color Intra predictor for the selected LM Intra mode.

The cross-color Intra mode according to the present invention may alsobe applied to a scalable coding system or multi-view coding system,where the current first-color block corresponds to a reconstructed blockin a reference layer or a reference view and the current second-colorblock corresponds to a to-be-coded or decoded block in a dependent layeror a dependent view.

Yet another embodiment of the present invention is directed to anon-transitory computer readable medium storing a computer-executableprogram, the computer-executable program, when executed, causing adecoder to perform the following steps: receiving neighboringreconstructed first-color pixels and current reconstructed first-colorpixels of a current first-color block; receiving neighboringreconstructed second-color pixels of a current second-color blockcollocated with the current first-color block; determining linear model(LM) parameters according to a linear model for one or more LM Intramodes, wherein the LM parameters for at least one of said one or more LMIntra modes is determined based on multiple rows of the neighboringreconstructed second-color pixels adjacent to a top boundary of thecurrent second-color block and a first part of the neighboringreconstructed first-color pixels, or multiple columns of the neighboringreconstructed second-color pixels adjacent to a left boundary of thecurrent second-color block and a second part of the neighboringreconstructed first-color pixels; receiving input data associated withcurrent second-color pixels of the current second-color block;generating a cross-color Intra predictor from the current reconstructedfirst-color pixels of the current first-color block using the LMparameters associated with a LM Intra mode selected from said one ormore LM Intra modes; and applying cross-color Intra prediction encodingor decoding to the current second-color pixels of the currentsecond-color block using the cross-color Intra predictor for theselected LM Intra mode.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates an example of derivation of chroma Intra predictionfor LM mode based on reconstructed luma pixels according to aconventional method for a 4:2:0 sampling format.

FIG. 2A-FIG. 2C illustrate an example of derivation of chroma Intraprediction based on reconstructed luma pixels according to Zhang et al.disclosed in JCTVC-G358.

FIG. 3A-FIG. 3C illustrate an example of multi-LM chroma Intra modesaccording to an embodiment of the present invention for a 4:2:0 samplingformat.

FIG. 4A-FIG. 4C illustrate an example of multi-LM chroma Intra modeswith multi-rows or multi-columns of neighboring reconstructed pixelsaccording to an embodiment of the present invention for a 4:2:0 samplingformat.

FIG. 5 illustrates an exemplary flowchart for LM chroma Intra predictionusing multi-rows or multi-columns of neighboring reconstructed pixelsfor deriving the LM parameters according to an embodiment of the presentinvention.

FIG. 6 illustrates an exemplary flowchart for LM chroma Intra predictionusing only top pixels or only left pixels of neighboring reconstructedpixels for deriving the LM parameters according to an embodiment of thepresent invention.

DETAILED DESCRIPTION

As mentioned before, in traditional LM chroma mode, both top and leftneighboring samples are used to derive LM parameters, as shown inFIG. 1. The chroma Intra prediction with additional LM modes as shown inFIG. 2A-FIG. 2C improve the performance. However, the method usingextended neighboring pixels causes higher computational complexityand/or more buffer requirement. In order to improve the codingperformance without causing noticeable impact on the computationalcomplexity and/or buffer requirement, embodiments of the presentinvention only use part of neighboring reconstructed samples in LMparameter derivation. For example, only left neighboring samples or onlytop neighboring samples are used to derive LM parameters in Left-Only orTop-Only LM chroma mode, as shown in FIG. 3B or FIG. 3C respectively inaddition to the regular mode with Left and Top neighboring pixels asshown in FIG. 3A.

In Left-Only or Top-Only LM chroma mode, the number of samples used toderive LM parameters is only half of that for the regular chroma Intraprediction mode with both Left and Top neighboring pixels. While themethod using Left-only or Top-only neighboring pixels can reduce thecomputational complexity of LM parameter derivation, the derived LMparameters may not be accurate enough. In a typical coding system, linebuffers may already be used in the system for other purposes, such asdeblocking filter. Another embodiment of the present invention re-usesthe existing buffers for LM parameter derivation without the need foradditional buffers. Re-using the line buffers in deblocking filterimplies that more sample lines or columns may be used for LM parameterderivation. Consequently, more accurate LM parameters can be obtainedwhile Left-only or Top-only neighboring pixels are used.

The deblocking filter for HEVC is applied to both horizontal andvertical block boundaries. For luma samples, the deblocking filter isoperated on four samples on each side of a boundary. For chroma samples(YUV420 format assumed), the deblocking filter is operated on twosamples on each side of a boundary. Therefore, four luma sample lines,four luma sample columns, two chroma sample lines, and two chroma samplecolumns may already be used in a HEVC system to implement deblocking.Therefore, these four luma sample lines, four luma sample columns, twochroma sample lines, and two chroma sample columns can be re-used in aHEVC system for chroma LM mode without increasing the bufferrequirement. FIG. 4A-FIG. 4C illustrate an example to re-use thedeblocking buffer to derive LM parameters for Multi-LM chroma modes withmultiple rows or columns according to an embodiment of the presentinvention. For the regular LM chroma mode using both Left and Topneighboring samples, the LM parameters for the LM chroma mode is shownin FIG. 4A, for a YUV420 color system which is the same as the examplein FIG. 1. For Left-Only or Top-Only LM chroma mode, two sample rows ortwo sample columns are used for LM parameter derivation, as shown inFIGS. 4B and 4C, respectively.

An example of syntax incorporating Multi-LM chroma modes is shown inTable 1. The existing HEVC syntax is modified to accommodate three LMchroma Intra modes for the chroma Intra prediction.

TABLE 1 Chroma Intra mode Codeword (Intra_chroma_pred_mode) 0 4 100Left + Top LM chroma mode 1010 Top-Only LM chroma mode 1011 Left-Only LMchroma mode 1100 0 1101 1 1110 2 1111 3

Another embodiment of the present invention uses distance-weighted LMchroma mode. The distance-weighted LM chroma mode blends two LMpredictors with different weighting values according to the distancesfrom the to-be-predicted chroma sample to the top and left blockboundaries. The two LM predictors are derived from left reconstructedboundary pixels and top reconstructed boundary pixels respectively.

According to the distance-weighted LM chroma mode, two sets of LMparameters for the current to-be-predicted chroma block are derivedfirst. The Left-only LM parameters {a_(L), b_(L)} are derived based onthe neighboring boundary pixels as shown in FIG. 3B. The Top-only LMparameters {a_(T), b_(T)} are derived based on the neighboring boundarypixels as shown in FIG. 3C.

After the LM parameters are derived, the to-be-predicted chroma sample Vis predicted by the collocated luma sample V_(col) in the current blockaccording to a linear model depending on the specific LM mode selected.If the Multi-LM mode selected corresponds to Left-only predictor (P_(L))or Top-only predictor (P_(T)), the Multi-LM predictor is derived asfollows:

$\quad\left\{ \begin{matrix}{{P_{L} = {{a_{L} \cdot {V_{col}\left( {x_{c\;},y_{c}} \right)}} + b_{L}}},} & {{{Left}\text{-}{only}\mspace{14mu}{predictor}},} \\{P_{T} = {{a_{T} \cdot {V_{col}\left( {x_{c\;},y_{c}} \right)}} + b_{T}}} & {{T{op}}\text{-}{only}\mspace{14mu}{{predictor}.}}\end{matrix} \right.$

In the above equations, (x_(c), y_(c)) specifies the location of theto-be-predicted chroma sample relative to the top-left sample of thecurrent chroma block. That is, x_(c) and y_(c) also indicate thedistance to the left block boundary and the top block boundary,respectively. Therefore, the distance-weighted LM predictor can bederived as follows.P=w·P _(L)+(1−w)·P _(T).

In the above equation, w is a weighting factor depending on x_(c) andy_(c) and w has a value from 0 to 1. If the to-be-predicted chromasample is closer to the left block boundary, w has a larger value. Onthe other hand, if the to-be-predicted chroma pixel is closer to the topblock boundary, w has a smaller value. The closer boundary samples areregarded as more trusted samples to derive LM parameters. Two examplesare provided as follows:

Example 1: Fine-Grained Weighted LM Predictor

In this example, each to-be-predicted sample has its own weighting valueaccording to its location,

$w = {\left( {1 - \frac{x_{c}}{x_{c} + y_{c}}} \right).}$

Example 2: Switched Weighted LM Predictor

Only two weighting values are used and the two values are switched bycomparing the distance to the top block boundary and the distance to theleft block boundary,

$w = \left\{ \begin{matrix}{0.75,} & {{{if}\mspace{14mu} x_{c}} < y_{c}} \\{0.25,} & {{otherwise}.}\end{matrix} \right.$

In yet another embodiment of the present invention, the Multi-LM chromamode uses multiple lines to increase LM parameter accuracy and usesdistance-weighted LM chroma mode as well.

While the inter-color (also called cross-color) based linear mode isshown for chroma Intra prediction using reconstructed luma samples, theinter-color based linear model may also applied to other color systems.For example, the color components may correspond to Red (R), Green (G)and Blue (B).

The Intra prediction for one color component using a linear model basedon another coded color component as disclosed above may be extended toscalable video coding or three-dimensional/multi-view coding. Forexample, a current block in a dependent view may be Intra predictedusing linear model based on a reconstructed color component in areference view. The reconstructed color component in the reference viewmay be the same color component as or different color component from thecurrent block. For example, the reconstructed color component in thereference view may correspond to luminance while the current block maycorrespond to luminance or chrominance.

The performance of a system incorporating embodiments of the presentinvention is compared with a system based on HEVC Test Model version10.0, where no LM chroma is used. A system incorporating a regular LMchroma mode is also included (indicated by LM in Table 2). The systemincorporating embodiments of the present invention include the 3-LMchroma mode (indicated by “3-LM” in Table 2) and the 3-LM chroma modecombined with multi-rows and multi-columns (indicated by 3-LM withMulti-Rows/Columns in Table 2). A negative number means the percentageof bitrate saved compared to the anchor system based on HM10.0. Thecomparisons are performed using various coding configurations, where AImeans all Intra coding, RA mean random access, LB means low delay B modeand LP means low delay P mode. As shown in Table 2, the systemincorporating 3-LM chroma mode achieved further improvement over theregular LM mode. The 3-LM chroma mode with multi-rows and multi-columnsachieves further improvement over the 3-LM chroma mode. The test videodata used has a YUV420 format.

TABLE 2 Class A AI RA LB LP and B Y U V Y U V Y U V Y U V LM −0.7 −8.2−4.5 −0.5 −9.6 −4.9 −0.2 −6.2 −3.8 −0.2 −7.5 −3.9 3-LM −0.8 −9.6 −5.6−0.4 −11.1 −5.8 −0.2 −7.1 −4.6 −0.3 −8.4 −4.7 3-LM with −0.8 −10 −6 −0.5−12 −6.4 −0.2 −7.8 −4.8 −0.4 −9.2 −5.1 Multi-Rows/ Columns

Further comparison results are shown in Tables 3-5 for other videoformats. The anchor system corresponds to a HEVC based system usingregular chroma Intra prediction without the LM chroma Intra mode.Compared to the anchor system, the system incorporating multiple LMchroma modes according to embodiments of the present invention achieves8.5%, 11.6%, 11.7% BD-rate reductions in AI-Main-tier, 6.9%, 8.3%, 9.4%BD-rate reductions in AI-High-tier, and 5.4%, 5.9%, 6.8% BD-ratereductions in AI-Super-High-tier respectively as shown in Table 3. WhenRGB444 format is used, the G component is treated as the luminance, andB and R are treated as chrominance components. Compared to thetraditional LM chroma mode, the multi-LM chroma mode achieves additional0.9% and 1.3% chroma BD-rate gains for AI-Main-tier, 0.6% and 1.0%chroma BD-rate gains for AI-High-tier, and 0.5% and 0.7% chroma BD-rategains for AI-Super-High-tier. For all Intra coding configuration, theencoding time increases 21%. However, the decoding time is roughlyunchanged as shown in Table 3.

TABLE 3 All Intra Main-tier All Intra High-tier All IntraSuper-High-tier Y/G U/B V/R Y/G U/B V/R Y/G U/B V/R RGB 4:4:4 −20.0%−18.6% −19.6% −15.7% −15.0% −15.8% −11.8% −11.3% −12.0% YCbCr 4:4:4−2.0% −8.5% −8.8% −2.3% −5.6% −7.9% −2.4% −3.8% −5.8% YCbCr 4:2:2 −1.8%−6.5% −5.5% −1.5% −3.4% −3.4% −1.1% −1.9% −1.9% Overall −8.5% −11.6%−11.7% −6.9% −8.3% −9.4% −5.4% −5.9% −6.8% Enc 121% 121% 121% Time [%]Dec 100% 100% 100% Time [%]

The comparison results for Random Access Main-tier and Random AccessHigh-tier are shown in Table 4. Compared to the anchor system, thesystem incorporating multiple LM chroma modes according to embodimentsof the present invention achieves 4.7%, 8.9%, 8.6% BD-rate reductions inRandom Access Main-tier, and 3.4%, 5.3%, 6.5% BD-rate reductions inRandom Access High-tier. The encoding time only increases slightly whilethe decoding time is about the same.

TABLE 4 Random Access Random Access Main-tier High-tier Y/G U/B V/R Y/GU/B V/R RGB 4:4:4 −11.4% −10.3% −12.4% −8.1% −6.6% −9.0% YCbCr 4:4:4−0.9% −8.6% −7.5% −0.9% −5.9% −6.6% YCbCr 4:2:2 −0.8% −7.6% −5.4% −0.7%−3.3% −3.5% Overall −4.7% −8.9% −8.6% −3.4% −5.3% −6.5% Enc Time 102%103% [%] Dec Time 100% 100% [%]

The comparison results for Low delay B Main-tier and Low delay BHigh-tier are shown in Table 5. Compared to the anchor system, thesystem incorporating multiple LM chroma modes according to embodimentsof the present invention achieves 1.7%, 4.2%, 3.9% BD-rate reductions inLow delay B Main-tier, and 1.2%, 2.1%, 2.6% BD-rate reductions in Lowdelay B High-tier. The encoding time only increases slightly while thedecoding time decreases 4%.

TABLE 5 Low delay B Low delay B Main-tier High-tier Y/G U/B V/R Y/G U/BV/R RGB 4:4:4 −4.3% −3.8% −4.7% −3.0% −2.1% −3.2% YCbCr 4:4:4 −0.2%−4.3% −3.1% −0.2% −2.5% −2.4% YCbCr 4:2:2 −0.3% −4.6% −3.7% −0.3% −1.7%−2.2% Overall −1.7% −4.2% −3.9% −1.2% −2.1% −2.6% Enc Time 102% 102% [%]Dec Time 96% 96% [%]

FIG. 5 illustrates an exemplary flowchart for LM Intra mode usingmulti-rows or multi-columns of neighboring reconstructed pixels forderiving the LM parameters according to an embodiment of the presentinvention. Neighboring reconstructed first-color pixels and currentreconstructed first-color pixels of a current first-color block arereceived from storage or a processor as shown in step 510. Thefirst-color component corresponds to the color component that isprocessed before the second-color component. For example, thefirst-color component may correspond to the luminance component. For anencoder, the neighboring reconstructed first-color pixels and thecurrent reconstructed first-color pixels of the current first-colorblock may be derived at the encoder. For example, a reconstruction loopin the encoder may be used to derive the neighboring reconstructedfirst-color pixels and current reconstructed first-color pixels of acurrent first-color block. For cross-color Intra prediction of a currentsecond-color block, the neighboring reconstructed first-color pixels andthe current reconstructed first-color pixels of the current first-colorblock have already been derived. The neighboring reconstructedsecond-color pixels of the current second-color block collocated withthe current first-color block are received as shown in step 520. The LMparameters (linear mode parameters) according to a linear model aredetermined for one or more LM Intra modes based on multiple rows of theneighboring reconstructed first-color pixels and the neighboringreconstructed second-color pixels adjacent to respective top boundaries,or multiple columns of the neighboring reconstructed first-color pixelsand the neighboring reconstructed second-color pixels adjacent torespective left boundaries as shown in step 530. Input data associatedwith the current second-color pixels of the current second-color blockare received as shown in step 540. For encoding, the input datacorresponds to second-color pixel data to be Intra coded. For decoding,the input data corresponds to coded second-color pixel data to be Intradecoded. Cross-color Intra predictor is generated from the currentreconstructed first-color pixels of the current first-color block usingthe LM parameters associated with a selected LM Intra mode as shown instep 550. Cross-color Intra prediction encoding or decoding is thenapplied to the current second-color pixels of the current second-colorblock using the cross-color Intra predictor for the selected LM Intramode as shown in step 560.

FIG. 6 illustrates an exemplary flowchart for LM Intra mode using onlytop pixels or only left pixels of neighboring reconstructed pixels forderiving the LM parameters according to an embodiment of the presentinvention. Neighboring reconstructed first-color pixels and currentreconstructed first-color pixels of a current first-color block arereceived from storage or a processor as shown in step 610. Theneighboring reconstructed second-color pixels of the currentsecond-color block collocated with the current first-color block arereceived as shown in step 620. The LM parameters for each of multiple LMIntra modes based on the neighboring reconstructed first-color pixelsand the neighboring reconstructed second-color pixels are determined asshown in step 630, wherein the LM parameters for at least one LM Intramode are determined only based on top pixels of the neighboringreconstructed first-color pixels and the neighboring reconstructedsecond-color pixels adjacent to respective top boundaries, or only basedon left pixels of the neighboring reconstructed first-color pixels andthe neighboring reconstructed second-color pixels adjacent to respectiveleft boundaries. Input data associated with the current second-colorpixels of the current second-color block are received as shown in step640. Cross-color Intra predictor from the current reconstructedfirst-color pixels of the current first-color block are generated usingthe LM parameters associated with a selected LM Intra modes as shown instep 650. Cross-color Intra prediction encoding or decoding is thenapplied to the current second-color pixels of the current second-colorblock using the cross-color Intra predictor for the selected LM Intramode as shown in step 660.

The flowcharts shown above are intended to illustrate examples ofimproved LM chroma mode for a video encoder and a decoder incorporatingembodiments of the present invention. A person skilled in the art maymodify each step, re-arranges the steps, split a step, or combine thesteps to practice the present invention without departing from thespirit of the present invention.

The above description is presented to enable a person of ordinary skillin the art to practice the present invention as provided in the contextof a particular application and its requirement. Various modificationsto the described embodiments will be apparent to those with skill in theart, and the general principles defined herein may be applied to otherembodiments. Therefore, the present invention is not intended to belimited to the particular embodiments shown and described, but is to beaccorded the widest scope consistent with the principles and novelfeatures herein disclosed. In the above detailed description, variousspecific details are illustrated in order to provide a thoroughunderstanding of the present invention. Nevertheless, it will beunderstood by those skilled in the art that the present invention may bepracticed.

Embodiment of the present invention as described above may beimplemented in various hardware, software codes, or a combination ofboth. For example, an embodiment of the present invention can be acircuit integrated into a video compression chip or program codeintegrated into video compression software to perform the processingdescribed herein. An embodiment of the present invention may also beprogram code to be executed on a Digital Signal Processor (DSP) toperform the processing described herein. The invention may also involvea number of functions to be performed by a computer processor, a digitalsignal processor, a microprocessor, or field programmable gate array(FPGA). These processors can be configured to perform particular tasksaccording to the invention, by executing machine-readable software codeor firmware code that defines the particular methods embodied by theinvention. The software code or firmware code may be developed indifferent programming languages and different formats or styles. Thesoftware code may also be compiled for different target platforms.However, different code formats, styles and languages of software codesand other means of configuring code to perform the tasks in accordancewith the invention will not depart from the spirit and scope of theinvention.

The invention may be embodied in other specific forms without departingfrom its spirit or essential characteristics. The described examples areto be considered in all respects only as illustrative and notrestrictive. The scope of the invention is therefore, indicated by theappended claims rather than by the foregoing description. All changeswhich come within the meaning and range of equivalency of the claims areto be embraced within their scope.

The invention claimed is:
 1. A method of cross-color Intra predictionbased on reconstructed pixels of another color component, the methodcomprising: receiving neighboring reconstructed first-color pixels andcurrent reconstructed first-color pixels of a current first-color block;receiving neighboring reconstructed second-color pixels of a currentsecond-color block collocated with the current first-color block;determining linear model (LM) parameters according to a linear model forone or more LM Intra modes, wherein the LM parameters for at least oneof said one or more LM Intra modes is determined based on multiple rowsof the neighboring reconstructed second-color pixels adjacent to a topboundary of the current second-color block and a first part of theneighboring reconstructed first-color pixels, or multiple columns of theneighboring reconstructed second-color pixels adjacent to a leftboundary of the current second-color block and a second part of theneighboring reconstructed first-color pixels; receiving input dataassociated with current second-color pixels of the current second-colorblock; generating a cross-color Intra predictor from the currentreconstructed first-color pixels of the current first-color block usingthe LM parameters associated with a LM Intra mode selected from said oneor more LM Intra modes; and applying cross-color Intra predictionencoding or decoding to the current second-color pixels of the currentsecond-color block using the cross-color Intra predictor for theselected LM Intra mode.
 2. The method of claim 1, wherein the LMparameters for said at least one of said one or more LM Intra modes aredetermined only based on said multiple rows of the neighboringreconstructed second-color pixels adjacent to the top boundary of thecurrent second-color block and said first part of the neighboringreconstructed first-color pixels, or only based on said multiple columnsof the neighboring reconstructed second-color pixels adjacent to theleft boundary of the current second-color block and said second part ofthe neighboring reconstructed first-color pixels.
 3. The method of claim2, wherein said multiple rows of the neighboring reconstructedsecond-color pixels comprise two rows of top pixels of the neighboringreconstructed second-color pixels adjacent to the top boundary of thecurrent second-color block.
 4. The method of claim 2, wherein saidmultiple columns of the neighboring reconstructed second-color pixelscomprise two columns of left pixels of the neighboring reconstructedsecond-color pixels adjacent to the left boundary of the currentsecond-color block.
 5. The method of claim 1, wherein said one or moreLM Intra modes correspond to three LM Intra modes, wherein the LMparameters for a first LM Intra mode are determined only based on saidmultiple rows of the neighboring reconstructed second-color pixelsadjacent to the top boundary of the current second-color block and saidfirst part of the neighboring reconstructed first-color pixels, whereinthe LM parameters for a second LM Intra mode are determined only basedon said multiple columns of the neighboring reconstructed second-colorpixels adjacent to the left boundary of the current second-color blockand said second part of the neighboring reconstructed first-colorpixels, and wherein the LM parameters for a third LM Intra mode aredetermined based on a single row of the neighboring reconstructedsecond-color pixels adjacent to the top boundary of the currentsecond-color block, a single column of the neighboring reconstructedsecond-color pixels adjacent to the left boundary of the currentsecond-color block, and a third part of the neighboring reconstructedfirst-color pixels.
 6. The method of claim 5, wherein said multiple rowsof the neighboring reconstructed second-color pixels comprise two rowsof top pixels of the neighboring reconstructed second-color pixelsadjacent to the top boundary of the current second-color block, and thefirst part of the neighboring reconstructed first-color pixels comprisestop pixels of the neighboring reconstructed first-color pixels near atop boundary of the current first-color block.
 7. The method of claim 5,wherein said multiple columns of the neighboring reconstructedsecond-color pixels comprise two columns of left pixels of theneighboring reconstructed second-color pixels adjacent to the leftboundary of the current second-color block, and the second part of theneighboring reconstructed first-color pixels comprises left pixels ofthe neighboring reconstructed first-color pixels near a left boundary ofthe current first-color block.
 8. The method of claim 5, wherein saidthird part of the neighboring reconstructed first-color pixels comprisestop pixels of the neighboring reconstructed first-color pixels near atop boundary of the current first-color block and left pixels of theneighboring reconstructed first-color pixels near a left boundary of thecurrent first-color block.
 9. The method of claim 5, wherein a syntaxelement is incorporated in a bitstream to indicate Intra prediction modeselected for the current second-color block, and wherein three differentvalues are assigned to the Intra prediction mode to indicate the threeLM Intra modes respectively.
 10. The method of claim 1, wherein thefirst-color pixels correspond to luminance pixels or green pixels, andthe second-color pixels correspond to chrominance pixels or blue/redpixels respectively.
 11. The method of claim 1, further comprisingretrieving the neighboring reconstructed first-color pixels and theneighboring reconstructed second-color pixels from a buffer associatedwith an in-loop filter for said determining LM parameters, wherein thebuffer stores the neighboring reconstructed first-color pixels and theneighboring reconstructed second-color pixels previously used for thein-loop filter.
 12. The method of claim 11, wherein said first part ofthe neighboring reconstructed first-color pixels and said multiple rowsof the neighboring reconstructed second-color pixels are retrieved fromthe buffer, and the LM parameters are determined based on the retrievedfirst part of the neighboring reconstructed first-color pixels and theretrieved said multiple rows of the neighboring reconstructedsecond-color pixels adjacent to the respective top boundaries.
 13. Themethod of claim 11, wherein said second part of the neighboringreconstructed first-color pixels and said multiple columns of theneighboring reconstructed second-color pixels are retrieved from thebuffer, and the LM parameters are determined based on the retrievedsecond part of the neighboring reconstructed first-color pixels and theretrieved multiple columns of the neighboring reconstructed second-colorpixels adjacent to the respective left boundaries.
 14. The method ofclaim 11, wherein the in-loop filter comprises a deblocking filter. 15.The method of claim 1, wherein the current first-color block correspondsto a reconstructed block in a reference layer or a reference view, andthe current second-color block corresponds to a to-be-coded or decodedblock in a dependent layer or a dependent view in a scalable codingsystem or multi-view coding system respectively.
 16. An apparatus ofcross-color Intra prediction based on reconstructed pixels of anothercolor component, the apparatus comprising one or more circuitsconfigured to: receive neighboring reconstructed first-color pixels andcurrent reconstructed first-color pixels of a current first-color block;receive neighboring reconstructed second-color pixels of a currentsecond-color block collocated with the current first-color block;determine linear model (LM) parameters according to a linear model forone or more LM Intra modes, wherein the LM parameters for at least oneof said one or more LM Intra modes is determined based on multiple rowsof the neighboring reconstructed second-color pixels adjacent to a topboundary of the current second-color block and a first part of theneighboring reconstructed first-color pixels, or multiple columns of theneighboring reconstructed second-color pixels adjacent to a leftboundary of the current second-color block and a second part of theneighboring reconstructed first-color pixels; receive input dataassociated with current second-color pixels of the current second-colorblock; generate a cross-color Intra predictor from the currentreconstructed first-color pixels of the current first-color block usingthe LM parameters associated with a LM Intra mode selected from said oneor more LM Intra modes; and apply cross-color Intra prediction encodingor decoding to the current second-color pixels of the currentsecond-color block using the cross-color Intra predictor for theselected LM Intra mode.
 17. A non-transitory computer readable mediumstoring a computer-executable program, the computer-executable program,when executed, causing a decoder to perform the following steps:receiving neighboring reconstructed first-color pixels and currentreconstructed first-color pixels of a current first-color block;receiving neighboring reconstructed second-color pixels of a currentsecond-color block collocated with the current first-color block;determining linear model (LM) parameters according to a linear model forone or more LM Intra modes, wherein the LM parameters for at least oneof said one or more LM Intra modes is determined based on multiple rowsof the neighboring reconstructed second-color pixels adjacent to a topboundary of the current second-color block and a first part of theneighboring reconstructed first-color pixels, or multiple columns of theneighboring reconstructed second-color pixels adjacent to a leftboundary of the current second-color block and a second part of theneighboring reconstructed first-color pixels; receiving input dataassociated with current second-color pixels of the current second-colorblock; generating a cross-color Intra predictor from the currentreconstructed first-color pixels of the current first-color block usingthe LM parameters associated with a LM Intra mode selected from said oneor more LM Intra modes; and applying cross-color Intra predictionencoding or decoding to the current second-color pixels of the currentsecond-color block using the cross-color Intra predictor for theselected LM Intra mode.